This invention relates to an improved outlet structure for a liquid processing vessel of a small media mill, often referred to as a sand mill. The outlet structure includes an improved screen assembly and the related construction for mounting the screen assembly with respect to the sand mill vessel.
Sand milling is a proven, practical, continuous, high production method of dispersing and milling particles in liquid to produce smooth, uniform, finely dispersed products. Some of the products in which the sand milling process is used includes paints, inks, dye stuffs, paper coatings, chemicals, magnetic tape coatings, insecticides, and other materials in which milling to a high degree of fineness is required.
In a typical sand milling process, the material or slurry to be processed is introduced at one end of the processing chamber or vessel and pumped through a small diameter grinding media while a rotor within the vessel agitates the media to insure proper milling and dispersion of small particles in the liquid or slurry being processed. Although the grinding media in years past was sand, more currently a small manufactured product of steel, glass or other materials is used.
The processed liquid exits from the vessel, but the grinding media must, of course, remain within the vessel. To accomplish this, the outlet structure typically includes a screen assembly which prevents the media from leaving the vessel while the processed liquid flows through the screen. U.S. Pat. No. 4,441,658, issued Apr. 10, 1984 describes a cup-shaped assembly that fits within a cylindrical wall leading to an outlet. The cup shape of the screen assembly provides a large filtering surface area. Other screen assemblies include segments forming a portion of a cylindrical wall. These screen components are typically formed of small diameter rods or strands which are welded at their intersections. A shortcoming of these welded constructions is that the screen become worn causing some of the strands of the screen break or the openings between the strands become large enough to allow passage of the grinding media. This requires early replacement of the screen.
Thus, a need exists for an improved longer lasting screen construction. The screen together with the vessel outlet structure must also be arranged so as to provide easy disassembly and cleaning or replacement of the screen components.
It is also desirable that the screen and outlet construction be sufficiently versatile to be useful for both vertically oriented sand mills and horizontally oriented sand mills. In this connection, there is also a need for periodic removal of the grinding media from the vessel. With vertically oriented vessels, this is accomplished fairly readily by means for a dump valve located at the lower end of the vessel. However, with horizontally oriented vessels the problem is more complex. Since the media lays along the bottom of the horizontal vessel for the entire length of the vessel, removal of the media or access to the screen or rotor has been troublesome. Prior horizontal mills are typically formed of sections which are bolted together and these sections must be disconnected. To access all of the media, usually results in media spilling out along the entire length of the vessel. To aid in this process, a large pan or tray may be utilized, with the result that the media must again be transferred to another container. The entire operation is messy and time consuming. Thus, a need exists for a simplified horizontal media mill which is easily cleaned and yet is long lasting and durable.